Variable q microwave cavity and microwave switching apparatus for use therewith



1961 P. Fv CHESTER ETAL 3,014,188

VARIABLE Q MICROWAVE CAVITY AND MICROWAVE SWITCHING APPARATUS FOR USE THEREWITH Filed Sept. 12, 1958 25 III/I I 22 Signal Generator 12! WITNESSES INVENTORS fi Peter F. Chester 8 Perer E. Wagner tates Patented Dec. 19, 195i fiice 3,014,188 VARIABLE Q MICROWAVE CAVITY AND MICRO- WAVE SWITCHING' APPARATUS" FDR USE H B WR i Peter F. Chester, Pittsburgh, and Peter E. Wagner, Forest Hills, Pa., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa, a corporation of Penn- .sylvania Filed Sept. 12, 1958, Ser. No. 760,584} 7 Ciaims. (C1. @3 83) This invention relates to variable Q microwave cavities and microwave switching apparatus, and more particularly to microwave switching apparatus employinga semiconductor element and especially suitable'for use at low temperatures for varying the Q of a resonant cavity.

Iii some types of two-level solid state masers, it is necessary to change the quality factor, i.e., the Q of a microwave cavity or a radio-frequency resonant structure from a high value to a low value and back again in rapid ]sequence. The required short time interval precludes the" use of any mechanical device, andmoreover the switch may have to operate at very low temperatures such, for example, as the temperature of liquid helium. A further requirement is that the switch must be able to function in the magnetic field required for maser action, having, for example, a field strength of 3000 gauss employed in an X-band maser, and this makes the use of ferrites complicated and undesirable. v

The instant invention is embodied in apparatus which meets the requirements set forth above in a simple and reliable manner. The apparatus is suitable for switching the Q of a microwave cavity at liquid helium temperatures and employs a semiconductor element, for example, an element or disc of germanium placed in a hole or bore in the cavity wall, the bore communicating with the cavity so that electromagnetic waves .in the cavity impinge against the semiconductor element. A pulse is applied to the germanium element from a pulse generator, the pulse causing avalanche breakdown, or, more precisely, low temperature breakdown due to impact ionization of neutral impurities, a phenomenon whereby the properties of a semiconductor material at liquid helium temperatures can be changed by the application of an electric field produced by a suitable pulse or other signal from those of a nonlossy dielectric to those of a lossy dielectric.

Accordingly, a primary object of the instant invention is to provide new and improved microwave switching apparatus.

Another object is to provide new and improved microwave switching apparatus especially suitable for use with m se s- S till another object is to provide new and improved microwave switching apparatus for use at a low temperature and in the presence of a strong magnetic field.

A further object is to provide a new and improved microwave cavity having a variable Q.

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' Th dean orms am's ev c v i s remainder of the way through the end portion 11, forming with the bore 12 of smaller diameter a lip or shoulder designated 13. Mounted in the bore 14 and preferably supported by the lip or shoulder 13 is a disc 15 of a suitable semiconductor material such, for example, as germanium, which may be soldered to the lip or to the walls of the bore, the disc 15 being of greater diameter than bore 12 so that it will preferably form a closed cavity, although the apparatus will work satisfactorily if there is a small space between the lip 13 and the adjacent side of disc 15 provided that electrical contact is still made to the edge of the germanium.

The inner conductor 19 of a coaxial cable is connected to the semiconductor disc 15 at electrode 18, the metallic outer conductor of the coaxialeable being designated 16 and fitting snugly in the bore 14, the end 17 of the coaxial cable being spaced a small distance from the germanium semiconductor disc 15, as shown. The outer conductor of the coaxial cable may be connected by lead 20 to ground 21.

The lead or conductor 19 is connected to a suitable signal generator, shown in block form at 22, to receive the output'therefrom. The type of waveform generator used depends on the particular application. Thus a pulse is used if one wants to pulse the cavity Q, but if a different type of Q-modulation were desired, one would apply a difierent waveform.

In the operation of the apparatus, as aforementioned use is made of the phenomenon of avalanche breakdownin an impure semiconductor material at low temperatures, in the order of 10 K. and below, the phenomenon also being known as low temperature breakdown due to impact ionization of neutral impurities, and being described in an article entitled Impact Ionization of Impurities in Germanium, by N. Sclar and E. Burstein, in The Physics and Chemistry of Solids, March 1957, Vol.2, pp. 1,-23. The term avalanche breakdown as employed herein and in the claims is defined as low temperature breakdown due to impact ionization of neutral impurities.

Where germanium is employed for the semiconductor, the impurity may consist of arsenic, antimony, or indium. The concentration of impurities has a range of permissible values, but must not be too low or the semiconductor will not become sufliciently lossy when the pulse or other breakdown signal is applied thereto, and the concentration must not be too high or the breakdown voltage required becomes excessive. For germanium the range lO f /cc, to lO /cc. is suitable. If a small voltage is applied to s e i e f impure rma i m a liquid helium temperatures, a current flows in accordance with Ohms law. As the voltage is gradually increased, a critical value of the voltage is reached at which the resistance of the germanium departs from Ohms law. Beyond this point the current through the germanium increases catastrophically with increasing voltage. For example, the resistance may fall from 10 ohms at a voltage corresponding to 5 volts/cm. to 10 ohms at a voltage corresponding to 6 volts/cm. of field strength, and the microwav e properties of the semiconductor change at the same time from those vof a non-lossy dielectric, having a value of K7 equal, for example, to 16, to those of a lossy dielectric.

In the apparatus shown, this change in resistance and loss factor of the semiconductor disc 15 at the avalanche breakdown point loads. the cavity 10 and lowers its Q. This lowering of Q may also be accompanied by'some change in the resonant frequency of the cavity 10.

As aforementioned, the disc of germanium 15 is solder ed, if desired, in place in the end wall 11 of'the cavity forming means and the semiconductor is broken down by a pulse or other signal from signal generator applied hereto, pv w y, o lead mean 19, the p s or ether sis- 3 nal being applied between the electrode 18 and the cavity wall 11.

The amount the Q of the resonant cavity is lowered upon avalanche breakdown of the semiconductor is determined by the parameters of the apparatus. In a typical condition of operation, where the radio-frequency employed falls in the X-band, and the temperature of the device is 4 absolute, the Q of the resonant cavity may be switched from a value of 8000 to a value of 4000. The response time of the switching apparatus is very short. A pulse of sufficient amplitude being provided, switching times in the order of one microsecond or less are developed.

Chamber means 23 containing liquid helium or other cooling gas, having chamber walls 24 and 25, is provided for maintaining the temperature of the semiconductor disc 15 and incidentally the adjacent portions of the resonant chamber forming means at very low temperatures approaching absolute zero, in the region of or below K., the walls being shown broken away for ease of illustration.

Where the variable Q resonant cavity and microwave switching apparatus are used in maser equipment, the whole of the apparatus may be maintained at a low temperature, so that the liquid gas chamber means 23 shown for maintaining semiconductor disc at a low temperature may be dispensed with.

Whereas the invention has been shown and described with reference to a resonant chamber having the semiconductor element mounted in an aperture in the wall thereof, other configurations in which the wave energy in the cavity impinges upon the semiconductor element are contemplated and are included in the scope of the invention.

Preferably, the coaxial transmission line is properly terminated by the semiconductor element.

The invention is not to be limited to the geometry described hereinabove, but the invention includes use as a waveguide attenuator in which case the semiconductor element is disposed inside the waveguide and a pulse or other signal applied thereto.

Whereas germanium was found to be the most suitable semiconductor material, silicon may be used as the semiconductor element in the instant invention.

Several other advantages are offered by the apparatus of the invention. No junctions are required in the semiconductor material for breakdown at low temperatures; the breakdown is purely a bulk effect. This is an advantage over junction-type switches which require provision for heat dissipation. The performance of the apparatus of the instant invention does not fall off at low frequencies down to zero frequency or direct current; this is an advantage over ferrites which do not operate well below about 1 kmc.

The upper frequency limit of the apparatus of this invention maybe extended somewhat by suitably reducing the collision time for carriers by appropriate doping.

The invention includes the use of an impure semiconductor element maintained at a low temperature, located in anelectrornagnetic wave energy field or path and having a signal applied thereto to provide a modulator of the electromagnetic wave energy.

The invention includes the use of an impure semiconductor element disposed in a microwave field to provide a microwave power level limiter, the microwave field itself causing breakdown when the field reaches or exceeds a predetermined value, thereby introducing losses which tend to reduce the power.

Whereas the apparatus has been shown and described with respect to a preferred embodiment thereof which gives satisfactory results, it will be understood that changes may be made and equivalents substituted without departing from the spirit and scope of the invention.

We claim as our invention:

1. Variable Q microwave apparatus suitable for operation at a low temperature approaching zero degrees absolute, comprising, in combination, means forming a resonant chamber for microwave energy, said chamber forming means having a bore extending through the wall thereof, the bore having a portion of relatively small diameter communicating with the chamber and a remaining portion of large diameter relative to the first named portion and forming a shoulder at the junction of the portion of small diameter and the portion of large diameter, means in said bore responsive to an electrical signal applied thereto for varying the Q of said chamber, said last named means including a solid element of impure semiconductor material composed entirely of a single type of semiconductivity of predetermined impurity concentration mounted in said portion of large diameter adjacent said shoulder in a manner to substantially close the bore without extending into said chamber and in a manner whereby the microwave energy in said chamber impinges through the bore portion of small diameter upon said element, and means for maintaining said element at said low temperature, said signal having a predetermined amplitude and causing a predetermined current to flow in said element, said element while maintained at said low temperature and when said predetermined current flows therein experiencing avalanche breakdown with a resulting substantial lowering of the resistance of the element'and a substantial increase in the loss in the semiconductor element, said increase in the loss in said element resulting in a decrease in the Q of said resonant chamber.

2. Variable Q microwave apparatus comprising, in combination, means forming a resonant cavity for microwave energy, said cavity forming means having a bore extending through the wall thereof, the bore having a portion of relatively small diameter communicating with the cavity and a remaining portion of large diameter relative to the first named portion and forming a shoulder at the junction of the portion of small diameter and the portion of large diameter, means in said bore responsive to an electrical signal of at least a predetermined amplitude for varying the Q of said cavity, said last named means including a solid semiconductor element composed entirely of a single type of semiconductivity of predetermined impurity concentration mounted in said portion of large diameter adjacent said shoulder in a manner to substantially close the bore without extending into said cavity, electromagnetic wave energy in the cavity impinging through the bore portion of small diameter against said semiconductor element, means for maintaining said element at a low temperature approaching absolute zero, signal generating means for generating said electrical signal, and circuit means connecting said signal generating means to said semiconductor element for applying the signal to said semiconductor element to cause a predetermined current to fiow in said element, said predetermined current flowing in said semiconductor element while said element is at said low temperature causing avalanche breakdown in said semiconductor element with an increase in the losses therein thereby causing a reduction in the Q of said resonant cavity.

3. Variable Q microwave apparatus comprising, in combination, means forming a resonant cavity for microwave energy, said cavity forming means having a bore extending through the wall thereof, the bore having a portion of relatively small diameter communicating with the cavity and a remaining portion of large diameter relative to the first named portion and forming a shoulder at the junction of the portion of small diameter and the portion of large diameter, means in said bore responsive to an electrical signal of at least a predetermined amplitude applied thereto for varying the Q of said cavity, said last named means including a solid semiconductor element composed entirely of a single type of semiconductivity of predetermined impurity concentration mounted in said portion of large diameter adjacent said shoulder in a manner to substantially close the bore without extending into said cavity, electromagnetic wave energy in said cavity impinging through the bore portion of small diameter against said semiconductor element, and means for maintaining said element at a low temperature approaching absolute zero, said signal causing a predetermined current to flow in said element, said element when the predetermined current flows therethrough experiencing avalanche breakdown with a resulting substantial decrease in the resistance in the element, said change in the resistance resulting in a decrease of the Q of said resonant cavity, said element rapidly recovering substantially its original resistance value at the termination of said current.

4. Variable Q microwave switching apparatus, comprising, in combination, means composed of electrically conductive material forming a resonant cavity for microwave energy, said cavity forming means having a bore extending through the wall thereof, the bore having a portion of relatively small diameter communicating with the cavity and a remaining portion of large diameter rela tive to the first named portion and forming a shoulder at the junction of the portion of small diameter and the portion of large diameter, means in said bore responsive to an electrical signal of at least a predetermined amplitude for varying the Q of said cavity, said last named means including a solid semiconductor element composed entirely of a single type of semiconductivity of predetermined impurity concentration mounted in said portion of large diameter adjacent said shoulder in a manner to substantially close the bore without extending into said cavity and in a manner whereby electromagnetic wave energy in said cavity impinges through the bore portion of small diameter against said semiconductor element, means for maintaining said element at a low temperature approaching absolute zero, signal generating means for generating said electrical signal, and circuit means including the wall of said cavity forming means connecting said signal generating means to said semiconductor element for applying the signal to said semiconductor element to cause a predetermined current flow in said element, said predetermined current flowing in said semiconductor element causing avalanche breakdown in said semiconductor element with an increase in the losses therein thereby causing a reduction in the Q of said resonant cavity.

5. Variable Q microwave apparatus adapted to operate at low temperature approaching absolute zero comprising, in combination, means forming a resonant chamber for microwave energy, said chamber forming means having an aperture in the wall thereof, means in said aperture responsive to an electrical signal of at least a predetermined amplitude for varying the Q of said chamber, said last named means including a solid semiconductor element composed entirely of a single type of semiconductivity of predetermined impurity concentration mounted in said aperture in a manner to substantially close the aperture without extending into said chamber, the microwave energy in said chamber impinging upon said semiconductor element, and means for maintaining said element at said low temperature, said signal of predetermined amplitude applied to said element causing predeterminedcurrent to flow in said element, said element While maintained'at said low temperature and when said predetermined current flows therein experiencing avalanche breakdown with a resulting substantial increase in the loss in. said element, said increase in the loss in said element resulting in a decrease of the Q of said resonant chamber.

6. In microwave switching apparatus, in combination, means forming a resonant chamber for microwave energy, said chamber forming means having an aperture in the wall thereof, means in said aperture responsive to an electrical signal of at least a predetermined amplitude for attenuating the wave energy in said chamber, said last "named means including a solid element of impure semiconductor material composed entirely of a single type of semiconductivit-y of predetermined impurity concentration mounted in said aperture in a manner to substantially close the aperture without extending into said chamber, electromagnetic wave energy in said resonant chamber impinging upon said element, means for maintaining said element at a low temperature approaching absolute zero, and circuit means connected to said semiconductor element for when energized applying said signal of predetermined amplitude to said semiconductor element to cause a predetermined current to flow in said element, said predetermined current flowing in said semiconductor element while the element is at said low temperature causing avalanche breakdown in said semiconductor element with a resulting increase in the losses therein thereby causing attenuation of the electromagnetic Wave energy in said resonant chamber.

7. Microwave switching apparatus comprising, in combination, resonant cavity forming means having an aperture in the wall thereof, means in said aperture responsive to an electrical signal of at least a predetermined amplitude for varying the Q of said cavity, said last named means including a solid element of impure germanium of predetermined impurity concentration mounted in said aperture in a manner to substantially close the aperture without extending into said cavity, electromagnetic wave energy in said cavity impinging against said germanium element, means for maintaining said element at a low temperature approaching absolute zero, and an electrode mounted on said germanium element and having said signal of predetermined amplitude applied thereto to cause a predetermined current to flow in said element, said predetermined current flowing in the element While the element is at said low temperature causing avalanche breakdown in said germanium element with an increase in the losses therein thereby causing a reduction in the Q of said resonant cavity, the termination of the signal resulting in the restoration of the Q of the cavity to its previous values.

References Cited in the file of this patent UNITED STATES PATENTS 2,579,327 Lund Dec. 18, 1951 2,582,205 Longacre Jan. 8, 1952 2,745,072 Goldstein et al. May 8, 1956 2,891,160 Leblond June 16, 1959 2,899,652 Read Aug. 11, 1959 2,918,572 Howell et al. Dec. 22, 1959 2,944,167 Matare July 5, 1960 FOREIGN PATENTS 439,457 Great Britain Dec. 6, 1935 202,013 Australia June 6, 1956 OTHER REFERENCES Shockley: Electrons and Holes in Semiconductors, D. Van Nostrand Co., Inc., New York, 1950, pages 29-31.

Solar et al.: The Physics and Chemistry of Solids, No. 1, March 1957, vol. 2, pages 1-23. 

